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From: | ANDREW BRAUN |
Subject: | [Discuss-gnuradio] [USRP N210] Timing Issues in GNURadio |
Date: | Thu, 5 Sep 2019 17:56:50 -0700 |
Hey everyone, I'm currently experiencing timing issues regarding synchronizing two USRP's. My current setup is the following: USRP 0 sends a packet with a known preamble (BPSK Modulated) to USRP1. The packet is formed in the following manner: <Preamble | Packet Header | Arbitrary Phase / Amplitude Modulation>. The preamble and packet header are both bpsk modulated. USRP 1 receives this packet, strips the preamble (using tags from a correlation estimator), and is left with just the arbitrary phase / amplitude modulation. My specific application is ML based, but my problem isn't. Essentially what I've noticed is, the received constellation of the "arbitrary" modulation I described above may look either noisy depending on when the USRP's start receiving or transmitting. Additionally, underflows and overflows will also desynchronize the way the constellation looks. So if the constellation looks "noisy" due to this issue, then an underflow may actually bring it back to synchronization. If the constellation looks great like the picture attached, then an underflow will bring the constellation to look bad. I believe this may be related to how I'm synchronizing these two USRP's. Right now I have the USRP blocks in GNURadio set to External Clock Source, External Time Source, and Synced to the same "Unknown PPS". Doing this I would assume the received constellation should be the same, but it isn't. I also thought that this may be a sample offset, but that's not the case either, as testing with a custom sample offsetting block did not fix the issue. Here is a picture of my flowgraph and the observed constellation differences. Note that the channel model / manual phase adjuster are not doing anything with the issue that I've seen, as bypassing them in gnuradio does not resolve the problem. (The observed problem is seen even on a constellation sink on the USRP Source). If anyone has any experience or knows about this, please let me know. P.S. I can also screen capture a video of the problem and show what's happening if that's easier. Thanks, Andrew Braun
PSK_Mod_Example.grc
Description: application/gnuradio-grc
SameSymbolTX'd.png
Description: PNG image
PSK_MOD_try1.png
Description: PNG image
MinimumReproducibleFlowgraph.grc
Description: application/gnuradio-grc
PSKExample.png
Description: PNG image
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